The present invention relates to a steel of the "3% to 5% by weight chromium" family as used for making tools that withstand heat and that work under high levels of stress, such as dies for stamping and forging, dies for wire drawing, molds for static casting or for casting under pressure, using various alloys such as alloys of aluminum, copper, or titanium.
Such steels are alloyed with chromium, molybdenum, and vanadium, elements which give them the required hot strength properties. More precisely, they are subdivided into three families of compositions having properties that are similar, such that these three families are used in the same applications. These are compositions that comprise the following alloying elements, with percentages expressed by weight:
5% chromium, 1.3% molybdenum, 0.5% to 1.3% vanadium, approximately; or PA1 3% chromium, 3% molybdenum, 0.5% vanadium, approximately; or else PA1 5% chromium, 3% molybdenum, 0.8% vanadium, approximately. PA1 the mechanical strength of material decreases smoothly with increasing temperature; and PA1 the material loses its initial properties which were imparted thereto by preliminary heat treatment because of the metallurgical transformations that take place under the combined effects of stresses and temperature giving rise to its mechanical strength weakening and then collapsing. PA1 C 0.3%-0.4% PA1 Cr 2.0%-4.0% PA1 Mo 0.8%-3.0% PA1 V 0.4%-1.0% PA1 W 1.5%-3.0% PA1 Co 1.0%-5.0% PA1 Si 0-1.0% PA1 Mn 0-1.0% PA1 Ni 0-1.0% PA1 0.33%-0.37% PA1 Cr 2.58%-3.50% PA1 Mo 1.20%-2.20% PA1 V 0.6%-0.9% PA1 W 1.8%-2.6% PA1 Co 1.5%-3.0% PA1 Si 0.2%-0.5% PA1 Mn 0.2%-0.5% PA1 Ni 0-0.3% PA1 P.ltoreq.0.008% PA1 Sb.ltoreq.0.002% PA1 Sn.ltoreq.0.003% PA1 As.ltoreq.0.005% PA1 the effects of these carbides on the properties and the structural stability are known only in broad outline.
Some of those steels are specified in the AISI nomenclature in the United States under the terms H11, H12, and H13, or in the DIN nomenclature in Germany under the names W1.2343, W1.2606, and W1.2344, and they are mentioned in French standard NF A 35-590.
In use, the surface of the tooling comes into contact with materials that are heated to high temperature, for example liquid aluminum at 600.degree. C.-750.degree. C. or steel that is to be forged and that has been preheated to 1200.degree. C.
Consequently, the surface of the tooling is itself raised to high temperature. As a result, temperature conditions are established within the tooling between its working portion which is subjected to heating and the remainder of the part which is cooled either by natural conditions or by forced cooling.
Under severe conditions of use implementing high surface temperatures and high levels of mechanical stress, a tool is destroyed quickly by two processes:
Thus, rapid or even catastrophic deterioration is observed of such tooling employed under severe conditions because the working surface softens, creeps, deforms plastically, and is subject to thermal fatigue.